The invention relates to a device for controlling the rotational speed of turbo-jet engines for aircraft, comprising a commanded value generator providing a commanded rotational speed signal which represents a commanded rotational speed of the high-pressure compressor of the turbo-jet engine, a tachometer generator which provides an actual rotational speed signal representing the actual rotational speed of the high-pressure compressor, means for forming the difference of these two signals as control deviation signal, a control device to which the control deviation signal is applied and which generates a control device output signal corresponding to a fuel flow rate and counteracting the control device output signal, a function generator which stores the relation between the fuel flow rates and the rotational speeds of the high-pressure compressor in accordance with the steady-state characteristics of the turbo-jet engine and provides a rate of fuel flow output signal as a function of a rotational input signal; and signal limiting means for limiting the signals determining the fuel flow rates such that sufficient distance from critical engine states, for example the surge line, is ensured.
The control of the rotational speed of turbo-jet engines throughout the whole range of rotational speeds requires a rather wide range of variation of the metered fuel flow. If a purely proportional-position control device (P control device) were used, this would result in a large deviation (proportional-deviation) between the commanded and the actual values of the rotational speed, after the rotational speed command has been executed; for a higher steady-state fuel flow is required with higher steady-state rotational speed, and a proportional-position control device can provide this higher fuel flow only if the control device output signal derived from the control deviation signal is greater than zero in the steady state. This, in turn, is only possible, if the control deviation is greater than zero in the steady state, i.e., the value of the actual rotational speed is smaller than the value of the commanded rotational speed. The proportional deviation is the smaller the larger the proportionality factor of the control device is. Only with an infinitely large proportionality factor could the proportional deviation be equal to zero. Since this cannot be accomplished in practice and for the additional reason that even increased proportionality factors already result in instability of the control loops, it is a conventional technique to connect an integral component in parallel thereto. This integral component integrates the permanent proportional control deviation until the control deviation signal equals zero, i.e., the commanded rotational speed is equal to the actual rotational speed. The use of an integrator, however, always has the effect of reducing the damping of the control loop. If certain requirements with respect to the damping must be met, it is necessary to try to relieve the control device from a major portion if its work by an open-loop control, since then the proportional and integral components of the control device can be applied with relatively low intensity only and the negative effects mentioned above can be reduced considerably.
Therefore, it is known to provide a function generator in which is stored the relation between the fuel flow and the rotational speed resulting from the characteristics of the turbojet engine for the steady state. An input signal indicative of a rotational speed causes an output signal from the function generator, said output signal representing the associated fuel flow for the steady-state ("steady fuel flow"). In the prior art device the actual rotational speed signal from the tachogenerator, which represents the actual rotational speed of the high-pressure compressor, serves as the input signal of the function generator. Thus an open-loop control signal is slaved to the respective actual rotational speed. This slaved open-loop control signal causes metering of a fuel flow which normally maintains the respective reached rotational speed independently of the control deviation. The control deviation signal is superposed on this slaved open-loop control signal.
When the rotational speed of a turbo-jet engine is to be run up in order to increase its power, an increased commanded rotational speed signal is provided by the commanded value generator, whereby the control deviation signal is increased and correspondingly a larger fuel flow is metered.
Now it could be that, with large rotational speed commands and correspondingly large control deviation signals and a slaved open-loop control signal additionally added, the fuel flow would be so large that the so-called "surge line" of the engine would be exceeded. In that event pulsations of the engine known as "surging" would occur, which would result in the destruction of the engine within a very short time (see, for example, Cohen, Rogers and Saravanamutto, "Gas Turbine Theory", Publisher Longman, London, 1972, pages 111-114). Therefore, it is known to limit the signal which determines the metered fuel flow by signal limiting means as a function of how close the respective engine operating parameters are to the surge line.
Furthermore, it is known to subject the control device output signal to limitations as a function of the engine operating parameters, for example pressures or temperatures. When such an engine parameter approaches or reaches a preselected value, the rotational speed control device output signal is eliminated by changing over to an associated limited value control loop.
It has now been found that in some cases, with the prior art slaving of the metered fuel flow to the actual rotational speed by means of the function generator, the transient time, until a commanded rotational speed is reached, is undesirably long.
It is the object of the invention to reduce the transient times in a device of the type initially defined for the control of the rotational speed of jet engines, i.e., to reduce the time required after a change of the commanded rotational speed signal to get the engine to the new commanded rotational speed. In accordance with the invention this object is achieved in that said input signal is the commanded rotational speed signal.
Thus in accordance with the invention, upon a change of the commanded rotational speed signal, the amount of fuel flow as a result of the output signal from the function generator is not slaved to the respective actual rotational speed; instead, the rate of change of the rotational speed is determined by the control deviation signal applied with low intensity and immediately a fuel flow will occur in an amount which corresponds to the new rotational speed dictated by the commanded rotational speed signal. Thereby, the jet engine, independently of the superposed control device output signal, is quickly speeded up to the new rotational speed and the transient time is reduced. The signal limiting means ensures that the increased fuel flow will not be sufficient to exceed the predetermined parameter limits of the engine but that the metered fuel flow is limited, if necessary. The engine is speeded up with a safety margin away from the surge line and without exceeding other limits. As the output signal from the function generator and corresponding to a fuel flow, but being now a function of the commanded rotational speed, only can be applied behind part of the signal limiting means for functional reasons, this output signal would ruin the effect of such limitation, in case the mentioned limitations become effective. This would result in an exceeding of the limits. In order to avoid this a further modification of the invention provides that the function generator receives the actual rotational speed signal as a second input signal and provides a second output signal which is indicative of the fuel flow associated with this actual rotational speed, and that, upon any of the said limitations becoming effective, this second output signal is superposed on the limited control device output signal instead of the output signal associated with the commanded rotational speed. This ensures that, when the limitations become effective, no larger signal may be applied at the series-connected junction of the function generator than that corresponding to the rotational speed just reached.
As inaccuracies may occur in the reproduction of the fuel flow versus rotational speed characteristic by the function generator, it is advantageous, also for this reason, that the control device output signal comprises a proportional component and an integral component, and that the integral component is switched off, when the control deviation signal exceeds a predetermined limit. The integral component is able to compensate for such inaccuracies. In the case of a limitation of the control device output signal, when a predetermined limit is approached or reached, the input to the integrator is switched off. This prevents the integral component from increasing undesirably during such a limitation, by which, among others, quick reduction of the control deviation signal is prevented. Otherwise a step-like signal, which is too high by the accumulated integral component, would be applied when the limitation is removed.
Furthermore, it is possible to supply the second output signal multiplied by a factor less than one to a maximum selector circuit to serve as extinction safety device, which circuit additionally receives the sum of the control device output signal, which may be limited, and the first or second output signal of the function generator superposed thereon. Thereby the second output signal from the function generator is utilized twice in a manner known per se.
In practice the control of the rotational speed of turbo-jet engines presents extraordinary problems since the controlled system is highly non-linear. To achieve a uniform control of the rotational speed throughout the whole range of rotational speeds of the jet engine, and a control largely unaffected by the non-linearities of the controlled system, it is furthermore advantageous if the control deviation signal is applied with a factor depending on the actual rotational speed and the total intake pressure of the turbo-jet engine to provide the control device output signal.